In recent years, an organic light-emitting diode has been attracting attention for the use purpose of, for example, a flat surface light source. The organic light-emitting diode has a configuration in which an emissive layer made up of an organic material is sandwiched between a pair of electrodes, that is, a cathode and anode. When a voltage is applied to the diode, electrons and holes are respectively injected from the cathode and anode into the emissive layer, and recombine in the emissive layer to generate excitons. When the excitons are radiatively deactivated, light emission is obtained.
An emitting dopant used in the organic light-emitting diode includes a fluorescent emitting dopant and phosphorescent emitting dopant. As for the fluorescent emitting dopant, a highly reliable dopant having a long life is obtained for all of blue, green, and red emitting dopants. However, since the fluorescent emitting dopant converts only singlet excitons into light emission, the maximum value of the internal quantum efficiency is limited to 25%. On the other hand, the phosphorescent emitting dopant which can convert both singlet and triplet excitons into light emission is expected to yield an internal quantum efficiency of nearly 100%. However, although a highly reliable dopant is obtained for green and red phosphorescent emitting dopants, development of a highly reliable blue phosphorescent emitting dopant has been slow.
In a white organic light-emitting diode which is expected to be applied to illuminations and backlights of displays, white light is obtained from those of three colors, that is, red, green, and blue. In such an organic light-emitting diode, when phosphorescent emitting dopants are used for all of red, green, and blue emitting dopants, a high luminous efficiency is expected, as described above. However, since the blue phosphorescent emitting dopant having a short life has to be used, a life time of the diode shortens, resulting in low reliability of the diode.
Hence, an attempt has been made to prepare a white organic light-emitting diode having a long life and high reliability by using a blue fluorescent emitting dopant having a longer life than the blue phosphorescent emitting dopant as a blue emissive layer, and using phosphorescent emitting dopants as a red and green emissive layer. In order to obtain white light emission by the organic light-emitting diode with such configuration, carrier recombination must take place in both the red and green emissive layer and the blue emissive layer, and the generated excitons have to be confined in the emissive layer. Various efforts to obtain white light emission have been made so far by devising the element configurations, materials to be used, and the like. However, excellent white light emission cannot be obtained. Even when white light emission is obtained, if the luminous efficiency is low, it is difficult to put the diode into practical use.